B Cell Stimulation through BCR and CD40 Modulates the Response of Chronic Lymphocytic Leukemia Cells to BAFF and APRIL.

The two TNF family proteins (B-cell activating factor [BAFF] and a proliferation-inducing ligand [APRIL]) and their three receptors (transmembrane activator and CAML interactor [TACI], B-cell maturation antigen [BCMA], and BAFF receptor [BAFF-R]) play a critical role in the process of differentiation, maturation and survival of normal B cells. Additionally, recent studies indicate that activation or inhibitory signals can modulate the sensitivity of normal B cells to BAFF and APRIL through the regulation of their receptors. In chronic lymphocytic leukemia (CLL), BAFF and APRIL have been shown to increase survival of neoplastic B cells in vitro. We investigated whether stimulation of CLL cells through the B cell receptor (BCR) or CD40 ligation could regulate the expression of BAFF-R, TACI and BCMA and enhance BAFF and APRIL sensitivity. Purified B cells were obtained from 23 CLL patients and nine healthy controls. Receptor expression was measured by flow cytometry at baseline and at 48 hours after stimulation with F(ab’)² antihuman IgM (10 μg/ml) and CD40L (500ng/ml) plus IL-4 (20ng/ml). Cell activation and viability, as assessed by labeling CD69 and Annexin V/TO-PRO-3, were evaluated at 48, and at 72 hours after co-stimulation with either soluble BAFF (100ng/ml) or APRIL (500ng/ml). Baseline analyses showed that BAFF-R was the most highly expressed receptor in CLL cells and normal B cells (Mean fluorescence intensity (MFI) ratios, 213.5 and 185.8, respectively). TACI and BCMA were also expressed in all CLL cells and normal B cells (MFI ratios TACI: 2.5 and 1.9; BCMA: 14.8 and 6.6, respectively), but at a significantly lower level than BAFF-R (p<0.001). Furthermore, BCMA MFI ratio was significantly higher in CLL than in normal B cells (p=0.015). After 48h of culture, an increase of all three receptors was observed in normal B cells in response to either BCR stimulation or CD40 ligation. In contrast, in CLL cells BCR stimulation induced almost no variation in the receptors expression in all cases. This was accompanied by a failure of cell activation and a significant decreased viability of CLL cells (from 36% to 24% p=0.013). By contrast, CD40 ligation in CLL cells induced a significant upregulation of TACI expression (p=0.007) and a significant reduction of BCMA (p=0.007), which correlated with an increase of CLL cell activation and viability (p<0.001). BAFF-R levels did not change. The addition of exogenous soluble BAFF or APRIL showed increase in the viability of normal B cells at 72 hours independently of whether cells were unstimulated or stimulated through the BCR or by CD40 ligation. In CLL cells, however, the viability was significantly increased in CD40-stimulated cells whereas in either unstimulated or BCR-stimulated CLL cells, the addition of BAFF and APRIL had a modest effect on viability (Table).

These findings indicate that stimulation of CLL cells through the BCR and CD40 modifies the sensitivity of CLL cells to respond to BAFF and APRIL which reflects the regulation of BCMA, TACI and BAFF-R. In contrast to normal B cells, CD40-ligation in CLL cells upregulated only TACI expression. The fact that the addition of CD40L plus IL-4 and BAFF increased viability in CLL cells while BAFF alone had almost no effect may be related to the ability of CD40 ligation to increase TACI expression. Although BCR stimulation failed to increase the expression of the receptors, co-stimulation by BAFF plus BCR increased viability in CLL cells.

No relevant conflicts of interest to declare.